1. Field of the Invention
This invention relates to a novel material that can be used for forming an electroconductive film and a method of forming an electroconductive film by using such a novel material as well as a method of manufacturing an electron-emitting device, an electron source and an image-forming apparatus.
A novel material according to the invention that can be used for forming an electroconductive film can also be used for forming a liquid crystal alignment film.
2. Related Background Art
Conventional electroconductive films that can be used for electric wiring and electrodes have a film thickness from several hundred to several thousand nanometers and are typically formed by means of a method involving the use of a vacuum apparatus such as vapor deposition or sputtering.
An electroconductive film formed by means of such a method can be used for the above identified applications as long as its electroconductivity is linearly proportional to its film thickness. If the electroconductivity is no longer linearly proportional to the film thickness, it is likely to change abruptly and go out of control. Additionally, with the above described method, the prepared film would be rather unstable in the initial stages of manufacturing in terms of chemical, physical and electric properties including electroconductivity. Thus, it has been extremely difficult with the above described conventional film forming method to control the electric properties of an electroconductive thin film particularly when the film thickness is less than several hundred nanometers and more particularly when the thickness is less than tens of several nanometers. The applications of such a thin electroconductive film include the electron-emitting region of surface conduction electron-emitting devices and the liquid crystal alignment film of liquid crystal type image-forming apparatuses. To begin with, an electron-emitting device will be described below.
There have been known two types of electron-emitting device; the thermionic cathode type and the cold cathode type. Of these, the cold cathode emission type refers to devices including field emission type (hereinafter referred to as the FE type) devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices and surface conduction electron-emitting devices. Examples of FE type device include those proposed by W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "PHYSICAL Properties of thin-film field emission cathodes with molybdenium cones", J. Appl. Phys., 47, 5248 (1976).
Examples of MIM device are disclosed in papers including C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32, 646 (1961).
Examples of surface conduction electron-emitting device include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of SnO.sub.2 thin film for a device of this type, the use of Au thin film is proposed in [G. Dittmer: "Thin Solid Films", 9, 317 (1972)] whereas the use of In.sub.2 O.sub.3 /SnO.sub.2 and that of carbon thin film are discussed respectively in [M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975)] and [H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 (1983)].
FIG. 20 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 20, reference numeral 1 denotes a substrate. Reference numeral 3 denotes an electroconductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually makes an electron-emitting region 2 when it is subjected to an electrically energizing process referred to as "energization forming" as described hereinafter. In FIG. 20, the thin horizontal area of the metal oxide film separating a pair of device electrodes has a length L.sub.1 of 0.5 to 1[mm] and a width W.sub.1 of 0.1[mm].
Conventionally, an electron emitting region 2 is produced in a surface conduction electron-emitting device by subjecting the electroconductive thin film 3 of the device to an electrically energizing preliminary process, which is referred to as "energization forming". In the energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of 1V/min is applied to given opposite ends of the electroconductive thin film 3 to partly destroy, deform or transform the film and produce an electron-emitting region 2 which is electrically highly resistive. Thus, the electron-emitting region 2 is part of the electroconductive thin film 3 that typically contains a gap or gaps therein so that electrons may be emitted from the gap. A surface conduction electron-emitting device that has been subjected to an energization forming process emits electrons from its electron-emitting region 2 when a voltage is applied to the electroconductive thin film 3 to make an electric current run through the device.
Now, a display apparatus comprising liquid crystal will be described. For example, to realize a display apparatus comprising liquid crystal in the twisted nematic mode (usually referred to as TN mode), an alignment film is formed on a pair of electrodes arranged on each of opposed surfaces of substrates and then subjected to a rubbing process where the film is rubbed along a direction perpendicular to that of alignment and liquid crystal is filled into the space between the films, the electrodes and the substrates. With such a process, the molecules of the liquid crystal is aligned to become electrically active so that the transmissivity of incident light that has passed through a polarizers is remarkably changed by the liquid crystal so that images are formed on the display apparatus.
Of electron-emitting devices of various types, surface conduction electron-emitting devices are particularly used for image-forming apparatuses having a large display screen because they are structurally simple and can be manufactured in a simple way. As a result of intensive research efforts, the inventors of the present invention have discovered that the energization forming process is particularly important in the manufacture of surface conduction electron-emitting devices having electrically excellent properties and that the use of thin film having a film thickness less than several hundred nanometers, preferably less than 20 nanometers, is particularly suitable for surface conduction electron-emitting devices. Thus, the inventors of the present invention have been engaged in inventing an improved method of preparing electroconductive thin film having a film thickness under the above defined limit. Specifically, electroconductive thin film made of a metal and/or the oxide thereof can be produced by applying a solution containing an organometallic complex to a substrate by means of a spinner and baking the substrate at high temperature in the atmosphere. While this technique of using a spinner is advantageous in that it can be produce a large electroconductive thin film because it does not involve the use of a vacuum apparatus as in the case of vapor deposition, it is accompanied by a difficulty with which a thin film having a uniformity of thickness is prepared because an organometallic complex is highly aggregative. While the highly aggregative tendency may be reduced by raising the concentration of hydrocarbon at the organic moiety of the organometallic complex, the energy required to attaching hydrocarbon to and detaching it from the organic moiety increases as the hydrocarbon concentration rises. As a result, the molten organomettalic complex can easily aggregate during the baking operation to produce electroconductive thin film having an uneven film thickness. On the other hand, an organometallic complex with a low hydrocarbon concentration at the organic part is likely to show an uneven distribution on a substrate to which it is applied and can partly be sublimated during the baking process to produce a thin film having an uneven film thickness. Particularly, while a very large electroconductive thin film having an uneven and optimal thickness has to be formed for preparing an electron source comprising a plurality of surface conduction electron-emitting devices arranged over a large area, the electron-emitting devices can perform unevenly in the operation of emitting electrons if the electroconductive thin film has an uneven distribution of film thickness and hence of electric properties.
Meanwhile, films of dielectric substances such as polyimide films having a film thickness less than tens of several nanometers, preferably less than 15 nanometers, are widely used as alignment films for image-forming apparatuses comprising liquid crystal. Such an arrangement is, however, accompanied by the problem of uneven film thickness, which by turn gives rise to an accumulated large electric charge on the part of the liquid crystal alignment film, hysteresis, afterimages and other transmittance-related troubles to reduce the image quality of the apparatus.
These problems will be dissolved if the electric resistivity of the polyimide film can be controlled to make it electroconductive to a certain extent. While a number of techniques may be conceivable for controlling the resistivity of the polyimide film, the most handy one will be controlling the impedance of the polyimide film by dispersing a metal and/or the oxide thereof into the film.
Polyimide can typically be obtained by chemically or thermally dehydrating and cyclizing (imidizing) polyamic acid that is a precursor of polyimide. Thus, a conceivable easy way of producing a polyimide film containing a metal and/or the oxide thereof in the form of dispersed fine particles may be to mix polyamic acid and a metal or an organic compound of the metal, form a film of the mixture on a substrate and then imidize the film. However, such a way is not practically feasible because the carboxylic group of the polyamic acid and the metal can easily react with each other for cross-linking to make gel and it is practically impossible to form a film of a polymer that has gelled.
An imide film containing a metal and/or the oxide thereof may alternatively be formed by preparing a mixture of soluble imide and the metal or an organic compound of the metal and laying the mixture on a substrate by appropriate means to form a multilayer structure. Then, gellation of the mixture can be avoided unless a group such as a carboxyl group is intentionally introduced into soluble polyimide to make the latter actively form a complex with metal. However, the structural types of polyimide that are compatible with this technique are narrowly limited to a small number.
A metal compound that would not allow two or more than two additional ligands to be coordinated may be used for a still alternative method of avoiding gellation of the mixture of polyamic acid and the metal or an organic compound of the metal. However, it is difficult to prepare and use a metal compound that has only one moiety for coordination when such additional ligands are prohibited from coordination. If no moiety is available for coordination in a metal compound, such a compound can hardly be mixed with polyamic acid to produce a desired mixture and the ingredients of the obtained mixture may more often than not be separated into different phases. Thus, it is very difficult to form a polyimide film containing a metal and/or the oxide thereof in an evenly dispersed state.